1. Technical Field
The present invention relates to an improvement of a pressure sensor utilizing a piezoelectric resonatorresonator which includes an electrode pattern formed on a piezoelectric substrate such as a crystal substrate and, more particularly, to a pressure sensor utilizing a piezoelectric resonatorresonator whose resonant frequency varies when pressure is applied in an axis direction.
2. Related Art
Conventionally, a pressure sensor utilizing a crystal resonatorresonator is known as a detection element used in gauges such as a water pressure gauge, an air gauge, and a differential pressure gauge. The crystal resonatorresonator includes an electrode pattern formed on a plate-like crystal substrate. By taking advantage of the characteristics of the crystal resonatorresonator that its resonant frequency varies when pressure is applied in the axis direction, the pressure sensor is made to detect pressure changes.
With the pressure sensor using the crystal resonatorresonator, the resonant frequency varies in approximate proportion to the applied pressure (showing a quadratic curve). Thus, by compensating the relation between the frequency variation and the applied pressure by using a quadratic equation, high-precision pressure measurement becomes possible.
However, in an attempt to realize the high-precision pressure measurement, problems occur that the structure becomes complicated and the manufacturing cost becomes higher. For example, a conventional pressure sensor as disclosed in FIG. 4 of JP-A-56-119519 includes: a case equipped with a pressure input orifice on the wall surface and maintained in a vacuum or inert atmosphere on the inside, an electro-deposited bellows whose one end opening side is fixed on the wall surface of the case, a force transmitting member connected to the other end of the bellows, a resonator supporting member joined to the force transmitting member via a flexure hinge, and a plate-like crystal resonator whose both ends are each supported by the force transmitting member and the resonator supporting member. The resonator supporting member is fixed, at its base, on the inner wall of the case and is equipped with the flexure hinge (a pivot) at the part joined to the force transmitting member.
With this conventional example, it is necessary to use the electro-deposited bellows having a very small spring constant and the flexure hinge with slim constricted parts. Because of high cost of these parts, the cost of the whole product becomes inevitably high. FIG. 7 of JP-A-56-119519 depicts another system in which two electro-deposited bellows are arranged in series between opposing wall surfaces of the case. However, similarly to the first described conventional example, this system also requires the electro-deposited bellows having a very small spring constant and the flexure hinge with slim constricted parts, thereby further increasing the cost.
Further, for price reduction, a system using inexpensive molded bellows and the force transmitting member having no flexure hinge is proposed. However, the pressure sensor of this type has a disadvantage in that, as the applied pressure increases, a bending stress component is applied to the crystal resonator in addition to the axial force. Thus, a linear frequency variation (the quadratic equation) cannot be obtained, and a relation of a cubic curve having a third-order coefficient is produced. Thus, the method of compensating the relationship between the frequency variation and the applied pressure by the quadratic equation suffers a disadvantage that it decreases the precision.
To solve these problems, the inventor of the present invention conceived a pressure sensor in which two inexpensive molded cylindrical bellows are arranged in series or concentrically in an airtight case and in which a crystal resonator is supported by a pedestal arranged between the ends of both bellows.
However, it is learned that the pressure sensor of this type has a disadvantage of not having enough strength against impact in a direction (an X-axis direction) perpendicular to the axis direction of the cylindrical bellows.